1. Field of the Invention
This invention pertains to dc-to-ac inverters and specifically to improvements in the protection and reliability of dc-to-ac inverters.
2. Description of the Prior Art
A brief history of dc-to-ac inverters is set forth in the Background of the Invention section of U.S. Pat. No. 3,946,301. The circuit which is described in that patent presents an inverter including several standard sections as the art had been developed at the time of that inverter development, viz., 1974 when the application that matured into that patent was filed.
Basically, the patent embodiment described therein includes a low power or control section and a high power section. The low power section includes a remote switch for connecting the low power section to a battery which is maintained charged by a battery charger, a square wave generator comprised of an oscillator and a divide-by-two circuit, and a pair of low voltage level transistor networks, one of which is operated by each of the complementary square wave outputs. Each of these networks includes a first and second stage transistor. This entire section controls at a low power level the operation of the high power section in push-pull fashion and supplies alternate drive current therefor.
The high power section of the '301 circuit includes a transformer for isolating the sections and for driving power switch driving transistors, which, in turn, control the operation of two banks of high power switch transistors. These high power transistor banks generate a high current square wave across the primary side of a ferro-resonant transformer, such transformer inherently converting its square wave input to a sine wave output.
The power transistor banks are each protected against overload by a resistor-capacitor network that senses the rise in collector voltage when such a transistor bank comes out of saturation and applies an input to the first stage of the related control or low voltage transistor, which results in turning off the second stage low level transistor and suspension of drive current to the driving transistor driving the transistor bank.
Should the square wave generator for some reason quit cycling so as to produce a constant high level output, the second stages of both low voltage or control transistors turn off because the capacitor in the base of these transistors no longer sense a changing voltage condition.
Several short comings of such a circuit have been observed. With respect to the low voltage or control network, when the remote switch is applied after a shut down condition, the square wave generator operates initially at a lower than design frequency as it comes up to speed, thereby providing a turn-on signal for too long a period of time, which tends to overdrive the power transistor banks. Second, the discrete components of the two-stage low voltage control transistors are not as reliable as integrated logic components and are more expensive. With respect to the high power section, the isolation transformer and the power transistor switches are not as reliable as field effect transistors have proven to be. Also, such components are more expensive. The banks of parallel power transistors have proven unreliable in that there is more stress put on some transistors in each bank than on others. Moreover, the imbalance requires precise control and selection of the ferro-resonant transformer shunts, windings, laminations and the like. When there is a failure of a transistor in a bank, the remainder of the transistors, instead of acting as a safety or back-up feature, actually also fail in domino fashion.
Additionally, there has been no way to provide synchronization to phase sensitive loads such as computers and the dimming control networks of HID lighting systems when such loads are switched from the normal ac power distribution line to the output of the inverter.
Finally, for large systems, it may be desirable to have several power sections of inverters at various locations. The isolation transformer and transistor drive arrangements are not sufficiently high impedance devices to permit modularizing for this purpose.
Therefore, it is a feature of the present invention to provide an improved dc-to-ac inverter with improved connections to a battery-regulated power source that maintains the square wave generator portion operable while switching only the last transistor control stage.
It is another feature of the present invention to provide an improved dc-to-ac inverter which incorporates electronic latches for reliably operating the control stages.
It is still another feature of the present invention to provide an improved dc-to-ac inverter not having an isolation transformer or a transistor drive circuit, but a cheaper and more reliable drive system.
It is yet another feature of the present invention to provide an improved dc-to-ac inverter having single stage power transistors instead of inherently unmatched banks of power transistors.
It is still another feature of the present invention to provide an improved dc-to-ac inverter having a protection circuit which monitors the current through the power transistors and not their out-of-saturation mode.
It is yet another feature of the present invention to provide an improved synchronizer and specifically an improved synchronizer that permits switching of phase sensitive loads from being controlled by the normal ac power distribution line to an inverter operating independently of such connection, such as would be the case when there is a line failure.
It is still another feature of the present invention to provide an improved dc-to-ac inverter having high impedance FET drivers in the power section thereof to permit modularizing the high power section so that multiple high power sections can be driven by a common control section.